A multimuscle state analysis of adult motor learning.

We introduce a new EMG state analysis to test two competing hypotheses about the role of muscle coactivity in learning a complex, multijoint reaching movement. Following Bernstein, one hypothesis is that as a task is learned, coactivity should decrease as degrees of freedom are released and limb stiffness is reduced. An alternative hypothesis is that as movement speed increases with learning, muscle coactivity should increase, possibly to stabilize joints against high inertial forces. Three participants performed a vertical reaching movement identical to that used by Schneider et al. We monitored the activity of four arm and shoulder muscles as participants completed 100 practice trials. Each frame of EMG activity was assigned to one of 16 possible combinations of the four monitored muscles based on an on-off activation threshold. This analysis yielded a time-based summary of muscle coactivity during the movement and across practice trials. Results of the state analysis supported the second hypothesis. As participants decreased their movement times over practice, coactivity increased - participants used more three- and four-muscle coactivity states. Changes were especially dramatic during the braking phase of the Up and Down portion of the vertical movement. When participants performed deliberately slow movements after speeded practice, three- and four-muscle coactivity was suppressed. We suggest that increased use of muscle coactivity may serve to counteract unwanted rotational forces generated during fast movements.